APU aka a battery. Enough to get ya down but it runs out eventually.
Not quite. The APU is a small engine, usually a turbine that runs off the same fuel as the main engines. It's generally on during takeoff and landing. It can be restarted in the air if need be. They do have batteries of course for electrical power should the APU fail as well. Plus hydraulic accumulators (Sort of like a capacitor or pressure tank for hydraulic fluid, or (less) like a battery), but those won't power the controls for very long at all.